In several other forums and guides I have read that Skirmish can be used to substitute Sneak Attack for the purpose of prerequisites of feats. However I cannot find anything in the rules about this. Is it true that Skirmish substitutes Sneak Attack in said case and where do the rules state this?
It appears that people (likely including me ⌐.⌐) are unconsciously expanding the rules about sneak attack and sudden strike in Complete Adventurer and Complete Scoundrel to also include skirmish. For reference, those rules:
Complete Adventurer pg. 8
Sudden Strike and Sneak Attack
For the purpose of qualifying for feats, prestige classes, and similar options that require a minimum number of sneak attack extra damage dice, treat the ninja’s sudden strike ability as the equivalent of sneak attack.
This sidebar appears in the section on the ninja, about sudden strike. The scout section does not have any corresponding sidebar about skirmish.
Complete Scoundrel repeats this statement in a sidebar on page 25 entitled “Skirmish and Sudden Strike,” where these two class features are reprinted since they are used in Complete Scoundrel prestige classes. The skirmish section, however, again does not indicate any such correspondence between skirmish and sneak attack.
Finally, Rules Compendium does not seem to repeat this rule at all, and refers to skirmish only in the section on precision damage, which does not address qualifying for anything, just how precision damage works.
So it looks like people are just unconsciously expanding the rules for sneak attack and sudden strike to include skirmish as well. And I think they are right to do so—skirmish and sudden strike are both inferior forms of precision damage, so it makes sense that options available to sneak attack would also be available to them. Skirmish’s damage is extremely low, and it could really use the benefit.
After looking through Complete Scoundrel, I found something that specifically states that Skirmish does not count as Sneak Attack for feat prerequisites:
Complete Scoundrel p.71/72
Although the skirmish class feature of a scout (Complete Adventurer) doesn’t count as sneak attack for the purpose of qualifying for feats, a scout with the Swift Ambusher feat (page 81) can combine sneak attack and skirmish extra damage for the purpose of qualifying for ambush feats. Even with this feat, a scout can’t sacrifice skirmish bonus damage to gain the benefit of an ambush feat.